"At what point do you have to maintain property so the government can use it to testify against yourself?"

For the duration of the border search. This guy is at risk because he caused the data to be deleted during a border search, when CBP asserts they can legally look. He would have been fine if he deleted data on the plane or after leaving the airport.

(This is my understanding of the government's position; personally I don't think this prosecution is constitutional)